overview: Hide when failing to take grab at end of startup animation

Otherwise keyboard input would be going to whatever window was
preventing us from taking the grab while it is obscured by the overview.

(cherry picked from commit 56478f21e6)

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2704>

js/ui/overview.js

@@ -692,6 +692,7 @@ var Overview = class extends Signals.EventEmitter {
 
             if (!this._syncGrab()) {
                 callback();
+                this.hide();
                 return;
             }